1. Field of the Invention
The present invention relates to tensioned shadow masks for color cathode ray tubes. More specifically, the present invention relates to methods and apparatuses for producing interchangeable shadow masks by tensioning the masks by stretching them to a fixed displacement.
2. Discussion of the Related Art
U.S. Pat. No. 4,902,257, ('257), issued Feb. 20, 1990, and assigned to the same assignee as the present invention, describes apparatus and process for manufacturing color cathode ray tubes of the flat tension mask type wherein shadow masks and front panels are respectively interchangeable during mask-panel assembly. According to the cited patent, position sensing means and a feedback control system are provided for applying controlled forces at a plurality of locations about the periphery of the mask, for the purpose of moving the mask to a desired position and stretching it to a desired size and shape for registration and assembly with the screened faceplate.
FIG. 1, corresponding to FIG. 12 of the '257 patent, depicts a prior art machine for applying controlled forces to a plurality of clamps gripping peripheral portions of the flat tension mask. The machine carries optical markers which cooperate with position-sensing apertures in the mask. It is the task of this machine to apply a distribution of forces to the mask such as to bring all position-sensing apertures in the mask into coincidence with their corresponding optical markers. This is achieved by feeding position error signals to a computer which calculates the required forces and feeds appropriate pulses to the stepping motors. Springs inserted between motors and clamps convert motor displacements into well defined forces.
The '257 patent also points out that if the photoetched masks were all exactly alike in thickness, elastic properties and detailed geometry, the forces to be applied to them to obtain a standard shape would always be the same, and no feedback would be required; but in practice there are unavoidable variations in thickness of the masks. To compensate for these variations, force adjustments are necessary, and these are controlled by a feedback loop operated by a computer system connected to each clamp.
It has now been found that a different, simpler mask stretching process is also capable of achieving the goal of interchangeability. This process eliminates the need for a feedback control system. Thus, position-sensing apertures in the mask, optical markers and photosensors in the assembly machine are no longer needed. The concept of applying feedback-controlled forces to the mask edges, embodied in the apparatus and method taught by the '257 patent, is set aside, and the concept of imposing a predetermined fixed displacement upon the mask edges, regardless of the required forces, is put in its place.
Forces acting on an elastic body are related to the displacement of its boundaries by Hooke's law. So long as the elastic limit is not exceeded, the two are proportional to each other. It may therefore seem, at first glance, that there is no important difference between controlling force and controlling displacement. If all masks were exactly alike, this would indeed be true. It is the nature of the variations between masks actually encountered which makes the distinction between force and displacement important and renders the process according to this invention suitable for large-scale production of flat tension mask cathode ray tubes.
Tension masks for mass-produced cathode ray tubes are presently made of steel. Nickel-iron alloys such as molybdenum permalloy may be used instead because of their superior mechanical and magnetic properties; such masks are described in U.S. Pat. No. 4,900,976 assigned to the same assignee. Typically, tension masks are 1 mil (one-thousandth inch) thick. Manufacturing tolerance on the thickness is typically plus or minus 10%. Experience shows that there are small thickness variations--considerably less than 10%--within each mask, as well as within one production lot of masks. In addition, there are significant thickness variations between different production runs which show up as corresponding variations in mask stiffness.
The variations just discussed change the forces that must be applied to the mask to stretch it to the desired shape and size. Therefore if, for example, electric stepping motors are used to produce the required forces, and springs are inserted between motors and clamps to produce an adjustable force, as suggested in the '257 patent, rather than a controlled displacement, then masks from different production runs will require different numbers of motor steps. A thicker mask, for example, requires the stepping motor to stretch its spring further so as to produce the extra force required by the extra thickness. Similarly, a small thickness variation between top and bottom halves of the mask will require different forces to be applied to the top and bottom clamps along the vertical edges of the mask. Therefore, a feedback system to control the stepping motors as described in '257 becomes necessary.
Other patents cited as background in the art include: U.S. Pat. Nos. 4,942,333; 3,573,528; 4,555,034; and 4,748,370.
This invention arrives at the desired result--to stretch a mask to the desired size and shape, and position it correctly, by a different, more straightforward procedure. Taking advantage of the fact that the unstretched photoetched masks are very much alike except for minor variations in thickness, in a preferred arrangement to be described, the initial position of the mask upon insertion into the assembly machine can readily be defined with great precision by retractable alignment pins cooperating with photoetched apertures in the mask. Means for selectively gripping a mask edge, e.g. clamps similar to those shown in the '257 patent but having hard-surface jaws (no elastomeric coating) are precisely positioned before the jaws are closed. These clamps are connected to a motive means, e.g. stepping motors, by links which are as rigid as possible in the direction of the pulling force exerted upon the mask, so that a given or fixed displacement of each stepping motor is transferred unchanged from the motor to the corresponding clamp and thus to the mask edge.
With this arrangement, the stepping motors are programmed to advance by a predetermined number of steps after the clamps are closed, thereby producing a predetermined fixed displacement of the mask edge around the circumference of the mask and stretching the mask to a predetermined size and shape. Since the mask was initially correctly positioned by means of the alignment pins and since all displacements from that initial position are made in accordance with a predetermined data file, the stretched mask not only has the correct size and shape, but is also located at the correct position.